Speaking from jail, Blackman said the killing, which happened in Helmand province in 2011 while he was serving with Plymouth-based 42 Commando, was a “split-second mistake”.

Lee was appalled by the trial and accused military authorities of “a failure of moral courage”, according to a report citing confidential legal documents.

The high-flying soldier, who went to school at King Edward’s in Birmingham, became a full colonel at the age of just 37 – the youngest member of the Corps to do so since the end of the Second World War.

He was Blackman’s de facto CO and was said to have resigned his Royal Marine commission at what he saw as a lack of support for front-line troops and a failure to take into account the pressures soldiers like Blackman faced in Afghanistan.

Lee was said to have believed “a serious breakdown of the sacrosanct relationship between command and commanded had occurred” before the incident in Helmand province on September 15, 2011.

Blackman was shown shooting the casualty in film from his helmet camera, telling the Taliban gunman to “shuffle off this mortal coil”.

Lee was said not to have been informed of the incident when it came to the attention of the authorities 12 months later.

He reportedly only found out about the court martial via the media despite having asked for a full role in the investigation.

A campaign is now taking place for Blackman’s sentence to be overturned, with an online petition reaching 100,000 signatures.

Lee, who also has an MBE, resigned his commission following the soldier’s conviction, only a year after attaining the rank of full colonel.

He was said to have written a letter in April 2014 referring to the risk of atrocities taking place on the battlefield due to factors including a breakdown in communication between front-line troops and their superiors.

Lee, now 40, reportedly wrote that Blackman faced “extraordinary factors” which should have been addressed by the chain of command.

The letter concluded: “Sgt Blackman was therefore sentenced by an authority blind to facts that offered serious mitigation on his behalf.

“The cause of this is a failure of moral courage by the chain of command, the burden of which is carried by the man under command.”

Lee has declined to comment.

A Ministry Of Defence spokesman said: “The MOD followed and supported the legal process throughout.

“This involved a full criminal investigation and a court martial where witnesses were called by both the independent Service Prosecution Authority and defence counsel.

“We respect the authority and decision of the court and would, of course, co-operate fully with any future legal process.